Artificial lift refers to the use of artificial means to increase the flow of liquids, such as crude oil or water or slurries containing fine particulates, from a production well or other downhole location. Artificial lift may be achieved by the use of a mechanical device inside the well, such as a pump, or by decreasing the weight of the hydrostatic column by injecting gas into the liquid some distance down the well (e.g. gas lift). Artificial lift is needed in wells when there is insufficient pressure in the reservoir to lift the produced fluids to the surface, but may be used in naturally flowing wells to increase the flow rate above what would flow naturally. Artificial lift systems include hydraulic pumps, electric submersible pumps (ESPs), gas lift systems, PCPs, rod pumps, and the like.
Progressive Cavity Pumps (PCPs) are commonly used positive displacement pumps in the oil and gas industry for artificial lift. These pumps are also known as progressing cavity pumps, eccentric screw pumps or cavity pumps. PCPs transfer fluid by means of the progress through the pump of a sequence of small, fixed shape, discrete cavities as a rotor is turned inside a stator. They are used as a form of artificial lift by operators especially for Coal Seam Gas (CSG) operations. CSG is also known as coal bed methane (CBM), coalbed gas or coal mine methane (CMM). These terms refer to methane absorbed into the solid matrix of coal in subterranean formations. CSG subterranean reservoirs are distinct from typical sandstone or other conventional gas reservoirs, because the methane is stored within the coal by an adsorption process. The methane is in a near-liquid state, lining the inside of pores within the core (the matrix). Open fractures in the coal (termed the cleats) can also contain free gas or can be saturated with water.
Hydraulic fracturing may be used to increase the fractures in a coal matrix to facilitate the production of CSG. However, hydraulic fracturing in a coal bed matrix may cause an increase in the generation of coal fines and debris during the fracturing procedure, particularly due to the fragile nature of the coal. Additionally, coal fines are hydrophobic in nature and tend to cause screen outs in the fracture generation which results in the hydraulic fracture not achieving its desired length. Coal fines are defined herein as having an average particle size of less than 50 microns, although fines may be as large as about 100 to about 200 microns. These coal fines may also not be produced during the flow back of the fracturing fluid, and may consequently reduce the conductivity of the fracture and reduce the production of CSG.
Another consequence of high levels of solids (e.g. coal fines that are displaced by CSG) in the broken fracturing fluid (which has had its viscosity reduced to the same as or near the viscosity of water) and/or produced water during the production of CSG is that the lifetime of the PCPs has been observed to progressively diminish from a life expectancy of about 24 months to approximately 12 months. The primary reasons for premature failure are associated with solids plugging.
More specifically, the three most common causes of premature PCP failure in CSG operations are: (1) overheating caused by solids plugging the intakes resulting in the pump being incompletely filled with fluid, which fluid also lubricates the pump, and thus causing the pump to overheat, (2) high torque loads due to solids settling above the pump, and (3) overheating due to gas intrusion through the pump leading to high temperatures causing the rotor seal elastomer to harden, swell, and crack.
It would be advantageous if methods and compositions could be discovered to prevent fine particulate solids, such as coal fines, from depositing in artificial lift systems, particularly pumps and pump intakes to thereby extend pump lifetimes.